Steel for cold punching and steel element for steel belt using the same

ABSTRACT

A steel used in a belt-type CVT that is superior in wear resistance and toughness, and a steel for cold punching, that provides the same. The steel satisfies the following: 10.8 [C]+5.6 [Si]+2.7 [Mn]+0.3 [Cr]+7.8 [Mo]+1.4 [V]≦13. The steel contains C within the range of 0.50 to 0.70%, Si within the range of 0.03 to 0.60%, Mn within the range of 0.50 to 1.00%, Cr within the range of 0.20 to 1.00%, Ti within the range of 0.01 to 0.10%, and B within the range of 0.0005 to 0.0050% by mass as required additional elements, P within the range of 0.025% or less and S within the range of 0.015% or less by mass as optional additional elements, and the remainder as Fe and unavoidable impurities.

TECHNICAL FIELD

The present invention relates to a steel for cold punching that is processed into an element for a steel belt used in a belt-type CVT of an automobile or the like, and the steel element.

BACKGROUND ART

In a belt-type continuously variable transmission (CVT) of an automobile or the like, a steel belt is wound between a pair of pulleys on an input side and an output side to transmit power. Such a steel belt comprises a structure wherein a plurality of chip-shaped elements (steel pieces) is assembled so as to overlap along a ring-shaped belt. This steel belt is inserted into a V-shaped groove of the output-side pulley and moves in the radial direction of the pulley when the groove width is changed, making it possible to continually adjust the rotation radius thereof and smoothly change the input-side and output-side pulley rotation ratio.

The steel element for a steel belt is continually driven while in contact with the V-shaped groove of the output-side pulley as described above, and therefore uses a steel with high hardness that is superior in wear resistance. In general, a steel with a relatively high carbon content, such as HS SKS95 (C: 0.80-0.90%, Si: 0.50% or less, Mn: 0.80-1.10%, P: 0.030% or less, 5: 0.010% or less, and Cr: 0.20-0.60% by mass) is used. A cold-rolled steel that includes a spheroidal carbide is cold-punched into an element shape, quenched and tempered from a temperature of Acm or higher on an equilibrium diagram, and provided with a tempered martensitic structure wherein a certain amount of insoluble carbides are dispersed.

To cold-punch a steel element, however, decreases productivity when using a steel with high hardness. Hence, manufacturing, methods whereby steel subjected to softening heat treatment is cold-punched and subsequently subjected to hardening heat treatment are considered. To prevent deformation of a steel element after punching, the hardening heat treatment should be performed at a relatively low temperature for a short period of time. In response to this, the present inventors investigated obtaining a steel having both high wear resistance due to high hardness and high toughness capable of withstanding contact by relative movement with the pulleys by focusing on a near eutectoid steel with the lowest temperature in an austenite single-phase stable-range on an equilibrium diagram, and performing hardening heat treatment at a temperature near the eutectoid composition point.

For example, Patent Document 1 discloses a high-carbon steel member that is a near eutectoid steel, having a high impact value greater than or equal to 25 J/cm² while maintaining a hardness of 600-900 Hv. Specifically, this prior art discloses a high-carbon steel member comprising a component composition of C: 0.60-1.30%, Si: <1.0%, Mn: 0.2-1.5%, P: 0.02%, S: ≦0.02%, Mo: ≦0.5% and V: ≦0.5%, by mass, wherein the insoluble carbides in the structure after quenching and tempering are caused to remain at a volume fraction V_(f) (volume %) of 8.5<15.3×C %−V_(f)<10.0 and the coarse insoluble carbides with a particle size of 1.0 μm or greater are restricted to two or less per observation area of 100 μm². Hence, the prior art states that adding Mo increases hardenability (quenchability) and toughness and forms a special carbide with Ni, thereby increasing wear resistance as well. Further, the prior art states that adding V refines the austenite grain, making it possible to increase wear resistance as well.

Further, Patent Document 2 discloses a near eutectoid steel, superior in toughness and fatigue resistance. Specifically, this prior art discloses a carbon steel comprising a component composition that includes C: 0.50-0.70%, Si: ≦0.5%, Mn: 1.0-2.0%, P: ≦0.02%, S: ≦0.02%, and Al: 0.001-0.10%, by mass, as well as one or two or more of V: 0.05-0.50%, Ti: 0.02-0.20%, Nb 0.01-0.50%, and Mo: ≦0.50%, by mass, wherein the spheroidizing ratio of the insoluble carbides in the structure after annealing, is 95% or greater, and coarse insoluble carbides with a particle size of 2.5 μm or greater are not produced. Hence, the prior art states that adding Mo increases hardenability, and adding V forms a carbonitride, increasing toughness.

PRIOR ART REFERENCE Patent Documents

-   Patent Document 1: Japanese Patent Application Publication No.     2006-63384 -   Patent Document 2: Japanese Patent Application Publication No.     2009-24233

SUMMARY OF INVENTION Problems To Be Solved By the Invention

Adding a rare metal such as Mo or V, as in the steel disclosed in Patent Document 1 and Patent Document 2, makes it possible to obtain a steel that is superior in wear resistance and toughness. Nevertheless, cost-wise, achieving roughly the same or a higher degree of wear resistance and toughness while decreasing the added amount of these rare metals is preferred.

The present invention has been made in view of such circumstances, and it is therefore an object of the present invention to provide a steel belt element used in a belt-type CVT of an automobile or the like that is superior in wear resistance and toughness while suppressing the amount of rare metals such as Mo and V, and a steel for cold punching that provides the same.

Means For Solving the Problems

The steel for cold punching, according to the present invention comprises a steel of a component composition that satisfies 10.8 [C]+5.6 [Si]+2.7 [Mn]+0.3 [Cr]+7.8 [Mo]+1.4 [V]≦13, given [M] as the percent by mass of the chemical element M, further comprising a component composition of C within the range of 0.50 to 0.70%, Si within the range of 0.03 to 0.60%, Mn within the range of 0.50 to 1.00%, Cr within the range of 0.20 to 1.00%, Ti within the range of 0.01 to 0.10%, and B within the range of 0.0005 to 0.0050% by mass as required additional elements, P within the range of 0.025% or less and S within the range of 0.015% or less by mass as optional additional elements, and the remainder as Fe and unavoidable impurities, wherein: a hardness of 88 HRB or less is achieved in a mainly ferrite+pearlite mixed structure dispersed fine carbides, when it is heated and held within a temperature range of an austenite single-phase and subsequently performed cooling at a predetermined rate.

According to such an invention, it is possible to favorably cold punch the steel for cold punching into a shape of a steel belt element of a belt-type CVT. Further, the steel for cold punching comprises a structure wherein fine carbides with a B nucleus are dispersed in a mainly ferrite+pearlite mixed structure. A predetermined quenching and tempering heat treatment makes it possible to suppress coarse carbides while providing high wear resistance as a steel element due to the dispersed structure of the fine carbides, thereby providing high toughness as a steel element as well.

The invention described above may be characterized by the suppression of coarse carbides with a circle-equivalent diameter of 0.5 μm or greater to 1.2×10⁵ carbides or less per 1 mm square in a sectional structure. According to such an invention, a predetermined quenching and tempering heat treatment makes it possible to suppress coarse carbides and provide high toughness as a steel element.

The steel belt element of a belt-type CVT according to the present invention is characterized by the provision of a quenching and tempering heat treatment after the steel for cold punching comprising any one of the inventions described above is cold punched into a predetermined shape, thereby providing a hardness of 640 Hv or greater.

According to such an invention, the steel element comprises high toughness as a steel element due to a structure that suppresses coarse carbides, while comprising high wear resistance as a steel element.

The invention described above may be characterized by the suppression of coarse carbides with a circle-equivalent diameter of 0.5 μm or greater to 1.3×10⁴ carbides or less per 1 mm square in a sectional structure. According to such an invention, the steel element comprises high toughness as a steel element due to a structure that suppressing coarse carbides, while comprising high wear resistance as a steel element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the manufacturing process of a steel element according to the present invention.

FIG. 2 is a diagram showing the sectional structure of a softening heat treatment.

FIG. 3 is a table showing the component composition of an embodiment and a comparison example.

FIG. 4 is a diagram showing the shape of a test piece of an impact test.

FIG. 5 is a diagram showing the method of the wear test.

FIG. 6 is a table summarizing the test results.

FIG. 7 is a diagram showing the impact ratio with respect to hardness after hardening heat treatment.

FIG. 8 is a diagram showing the composition profile of insoluble carbides.

FIG. 9 are photographs of a sectional structure showing the advance of a crack.

FIG. 10 is a graph showing the results of the wear test.

FIG. 11 is a graph showing the results of the wear test.

FIG. 12 is a diagram showing the observed quantity per insoluble carbide size after softening heat treatment.

FIG. 13 is a diagram showing the observed quantity per insoluble carbide size after hardening heat treatment.

MODE FOR CARRYING OUT THE INVENTION

The following describes the manufacturing method of a steel element for a steel belt of a belt-type CVT as one embodiment according to the present invention, based on FIG. 1.

First, a thin steel plate comprising a predetermined component composition near an eutectoid composition that includes a predetermined amount of B and Ti is subjected to softening heat treatment (S1) to make a punching process easier as described later. According to such a softening heat treatment, the thin steel plate is heated to a relatively low temperature that is in the austenite single-phase stable-temperature range, that is a temperature that is 20-30° C. higher than lines A3 and Acm, maintained at that temperature for a predetermined period of time, and cooled at a predetermined rate. According to this softening heat treatment, it is possible to obtain a steel for cold punching wherein insoluble carbides are finely dispersed by B in the component composition. This cold punching steel comprises favorable cold punchability making it possible to readily process a steel element shape.

Hence, as shown in FIG. 2, in the thin sheet plate comprising a pearlite structure, B is particularly dispersed in the cementite section in the pearlite structure (refer to FIG. 2 (a)). When this thin steel plate is heated and held in the austenite single-phase stable-temperature range, the thin steel plate changes to an austenite single phase (refer to FIG. 2 (b)). Before the thin sheet plate completely changes to the austenite single phase and when the temperature once again gradually decreases to the ferrite stable-temperature range, first the carbons that cannot dissolve in the ferrite precipitate as carbides, with some of the carbides precipitating with the dispersed B as the precipitation nucleus (refer to FIG. 2. (c) and FIG. 8 as well). When the temperature continues to decrease, the structure becomes one wherein the carbides are finely dispersed in a mixed structure of coarse pearlite grains and ferrite grains (refer to FIG. 2 (d)).

That is, to make the temperature decrease to the ferrite stable-temperature range while maintaining the dispersion of B as is without causing B aggregation, the decrease in the maintained temperature is started before the steel is completely changed to the austenite single phase at a relatively low temperature near the line A3 and the line ACM. Note that including Ti in combination with B causes Ti to preferentially generate nitrides with N rather than B and suppress generation of B nitrides, thereby maintaining the dispersion of B.

Once again with reference to FIG. 1, the cold punching steel is punched into a predetermined element shape, obtaining the steel element (S2).

Furthermore, a hardening heat treatment is performed to impart wear resistance and the like to the steel element obtained by punching (S3). That is, quenching and tempering are performed. Note that, to prevent deformation of the steel element comprising the thin steel sheet, this heat treatment is preferably performed at a relatively low temperature for a short period of time. That is, similar to the softening heat treatment (S1), the thin steel sheet is heated to and maintained at a relatively low temperature, which is in the austenite single-phase stable-temperature range, and quenched. In such a case, fine carbides wherein B is dispersed in the nucleus by the softening heat treatment are maintained. With this arrangement, it is possible to obtain a steel belt element used in a belt-type CVT of an automobile or the like with superior wear resistance and toughness.

Next, an evaluation regarding the hardness and the like required by the cold punching steel in order to cold punch the steel into a steel belt element was conducted, and an evaluation regarding the mechanical characteristics (toughness and wear resistance) required as a steel belt element when hardening heat treatment was provided to this steel for cold punching was conducted. These are described using FIGS. 3-5.

First, the present inventors achieved the following empirical equation for the relationship between component elements and hardness when adjusting a component composition, such as JIS SKS95, to ensure achievement of a hardness suitable for cold punching.

H₁=10.8 [C]+5.6 [Si]+2.7 [Mn]+0.3 [Cr]+7.8 [Mo]+1.4 [V]+75   (Equation 1)

Hence, target values of component compositions were first selected to ensure that predetermined hardness values described later were achieved by Equation 1 and, once the steel was manufactured, steels of the component compositions of embodiments 1-10 and comparison examples 1-11 shown in FIG. 3 were obtained. In the component compositions of embodiments 1-10 and comparison examples 1-11 of FIG. 3, Mo was added in comparison example 3 and V was added in comparison example 4 in accordance with target values, but were not intentionally added based on target values and were detected as impurities in the other embodiments and comparison examples.

The following describes the manufacturing method of a test piece used for evaluation. First, 150 kg of a master alloy was melted by a vacuum induction furnace, and ingots comprising the component compositions shown in FIG. 3 were obtained.

Next, the ingot was maintained at 1200° C. for three hours, a portion thereof was cut off and hot forged into a round bar of a substantially cylindrical shape with a 25-mm diameter. Note that the temperature after forging completion was 900° C. or higher. Next, the round rod was maintained at 840° C. for 60 minutes, air-cooled, and normalized. Further, the remainder of the cut-off ingot was hot-rolled to 3.5 mm, similarly normalized, and then cold-rolled to 1.5 mm to obtain a rolled steel.

The rolled steel was then subjected to heat treatment whereby the steel was maintained at 760° C. for one hour, slow-cooled to 650° C. at 10 ° C./hour, and subsequently air-cooled, as the above described softening heat treatment (S1). The steel was then suitably polished, etc., made into test pieces for structure observation, and measured for Rockwell hardness, the number of insoluble carbides, and the like.

The round rod, after subjected to heat treatment resembling the same softening heat treatment (S1), was maintained at 800° C. for 30 minutes and then subjected to quenching and tempering whereby the round bar was quenched in a 70° C.-oil bath and then tempered at 180° C. for 120 minutes, as the above described hardening heat treatment (S3). A portion of the round rod after heat treatment was then cut off and processed as a sub-size impact test piece 1 of a shape such as shown in FIG. 4, and a wear test piece 13 of a substantially right-angled parallelepiped block shape with a width, height, and thickness of 15.75 mm, 10.16 mm, and 6.35 mm, respectively, for the wear test described later. Note that the test piece 13 was suitably polished, etc., and measured for Vickers hardness, the number of insoluble carbides, and the like as a test piece for structure observation.

The average value of the Rockwell hardness measured at five arbitrary points using a commercial Rockwell hardness tester was established as a measured value H₂. Note that, since the ease of cold punching based on a hardness less than 88 HRB differs empirically, the cold punchability (machinability) was evaluated as favorable (O) when the measured value H₂ was less than 88 HRB, and as unfavorable (X) when the measured value H₂ was greater than or equal to 88 HRB in FIG. 6.

Vickers hardness was measured at five arbitrary points at positions where the depth from the front surface was approximately 25 μm in the cross-section of the wear test piece 13 using a commercial Vickers hardness tester, and the average thereof was established as a measured value H₃.

The impact test was conducted using a commercial Charpy impact tester. Note that the impact ratio in FIG. 6 is a ratio with respect to the measured value using the test piece of comparison example 3. Toughness was evaluated as favorable (O) in a case where this impact ratio was greater than or equal to 1, and as unfavorable (X) in a case where the impact ratio was less than 1.

The wear test was conducted b a block-on-ring method using a wear tester 10 such as shown in FIG. 5. Specifically, the wear test piece 13 was caused to contact a ring 11 that was partially immersed and rotated in a tank 14 that stores an oil 12 at 110° C. at a load of 1200 N, and the amount of wear was measured at a relatively slipped distance of 3000 m. Note that the slip velocity of the wear test piece 13 with respect to the ring 11 is 0.05 m/sec. Further, the ring 11 is a ring body with an outer diameter of 35 mm and a thickness of 8.74 mm, comprising a steel in which an SCM420 carburized quenched and tempered steel was refined. to a hardness of about 750 Hv. The wear ratios of FIG. 6 are ratios of the measured cross section of the worn area of the wear test piece 13 with respect to the wear surface area of comparison example 3. Wear resistance was evaluated as favorable (O) in a case where this wear ratio was less than 1, and as unfavorable (X) in a case where the wear ratio was greater than or equal to 1.

The number of insoluble carbides was measured by image analysis of a sectional structure. The number of insoluble carbides with a circle-equivalent diameter of 0.50 μm or greater that exists per 100 μm square was converted to the number that exists per 1 mm square.

The results are summarized in FIG. 6.

First, an estimated value H₁ of the hardness calculated from each of the component compositions of FIG. 3 using the-above described equation 1 and the measured value H₂ were found to be well in agreement. This indicates that the effect on hardness can be estimated by equation 1 even in the embodiment in which B and Ti were added.

According to each of the embodiments 1-10, the hardness H₂ after softening heat treatment (S1) (hereinafter referred to as the “hardness after softening heat treatment”) is less than 88 HRB, resulting in superior punchability. On the other hand,, the hardness H₃ after hardening heat treatment (S3) hereinafter referred to as the “hardness after hardening heat treatment”) is substantially the same as or less than or equal to that of comparison example 3, the impact ratio is greater than or equal to 1, and the wear ratio is less than 1. That is, compared to the prior art steel, the toughness and wear resistance are greater than or equal to the prior art value.

As a reference, according to comparison example 1 in which the C content was increased without adding Mo compared to comparison example 3, the hardness H₂ after softening heat treatment is a high 89.0 HRB, resulting in inferior punchability. Further, while the hardness H₃ after hardening heat treatment is high, the impact ratio is less than 1 and the wear ratio is greater than 1. That is, comparison example 1 is inferior to comparison example 3 in terms of toughness and wear resistance. The number of insoluble carbides after hardening heat treatment is extremely greater than that of comparison example 3, suggesting that wear resistance decreased.

According to comparison example 2 in which the C content was decreased without to adding Mo compared to the prior art steel, the hardness H₃ after hardening heat treatment is a low 537 Hv and the impact ratio is eater than 1 while the wear ratio is an extremely high 5.58. That is, comparison example 2 is extremely inferior in wear resistance compared to the prior art steel.

According to comparison example 4 in which V was added in place of Mo compared to the prior art steel the toughness and wear resistance are substantially the same as those of comparison example 3.

According to comparison examples 5 and 7 in which the Si content and Mn content were increased without adding Mo compared to the prior art steel, the hardness H₂ after softening heat treatment exceeds 88 HRB and tends to be inferior in cold punchability as a steel for cold punching.

On the other hand, according to comparison example S in which the Mn content was decreased without adding Mo compared to the prior art steel, the hardness H₂ after softening heat treatment is low, and the cold punchability as a steel for cold punching is favorable. Nevertheless, the wear ratio is extremely greater than 1 and the wear resistance is greatly inferior to that of the prior art steel.

According to comparison example 9 in which the Cr content was increased without adding Mo compared to the prior art steel, the cold punchability as a steel for cold punching is favorable. However, the impact ratio is less than 1 and the wear ratio is greater than 1, resulting in a toughness and wear resistance inferior to those of the prior art steel.

According to comparison example 10 in which the Cr content was decreased without adding Mo compared to the prior art steel, the wear ratio is greater than 1, resulting in a wear resistance inferior to that of the prior art steel.

On the other hand, according to embodiment 10 in which B and Ti were added in place of Mo compared to the prior art steel, the steel comprises favorable cold punchability as a steel for cold punching, with a high impact ratio of 1.22 and a low wear ratio of 0.65. That is, the steel comprises favorable toughness and wear resistance.

According to embodiment 1 as well in which B and Ti were added in place of Mo, and Mn was added while decreasing the C content compared to the prior art steel, the steel comprises favorable punchability as a steel for cold punching, an extremely high impact ratio of 1.40, and particularly superior toughness.

According to embodiment 2 as well in which B and Ti were added in place of Mo and the Cr content and P content were increased compared to the prior art steel, the steel comprises favorable punchability as a steel for cold punching. Further, both toughness and wear resistance are greater than or equal to those of the prior art steel.

According to embodiment 3 as well in which B and Ti were added in place of Mo and the Mn content was decreased compared to the prior art steel, the steel comprises favorable punchability as a steel for cold punching, a high impact ratio of 1.24, and a low wear ratio of 0.68, resulting in favorable toughness and wear resistance.

According to embodiment 4 as well in which B and Ti were added in place of Mo and the Si content was increased compared to the prior art steel, the steel comprises favorable punchability as a steel for cold punching, a high impact ratio of 1.20, and a low wear ratio of 0.65, resulting in favorable toughness and wear resistance.

According to embodiment 5 as well in which B and Ti were added in place of Mo and the C content was increased while decreasing the Si content compared to the prior art steel, the steel comprises favorable punchability as a steel for cold punching. Further, both toughness and wear resistance are greater than or equal to those of the prior art steel.

According to embodiment 6 and 7 as well in which B and Ti were added in place of Mo and the B content and Cr content were decreased compared to the prior art steel, the steel comprises favorable punchability as a steel for cold punching. Further, the toughness of the steel as a steel belt element is greater than or equal to that of the prior art steel, and the wear resistance is favorable.

According to embodiments 8 and 9 as well in which B and Ti were added in place of Mo and the S content and Ti content were increased compared to the prior art steel, the steel comprises favorable punchability as a steel for cold punching, and superior toughness and wear resistance as a steel belt element.

Now, according to comparison example 6 in which B and Ti were not added compared to embodiment 10, the steel comprises favorable punchability as a steel for cold punching. However, the impact ratio is less than 1 and the wear ratio is greater than 1 resulting in both a toughness and wear resistance inferior to those of the prior art steel. In particular, the toughness and wear resistance are greatly inferior to those of embodiment 10.

Further, according to comparison example 11 as well in which Ti was not added compared to embodiment 10, the steel comprises favorable punchability as a steel for cold punching. However, the impact ratio is less than 1 and the wear ratio is greater than 1, resulting in both a toughness and wear resistance as a steel belt element inferior to those of the prior art steel. In particular, the toughness and wear resistance are greatly inferior to those of embodiment 10.

The following describes the tendencies achieved from each of the results of the above embodiments and comparison examples.

As shown in FIG. 7, in a graph that compares the impact ratio with respect to the hardness H₃ after hardening heat treatment, embodiments 1-10 in which B and Ti were added are in substantially the same position as comparison example 3 in which Mo was added and comparison example 4 in which V was added. At least, embodiments 1-10 are in a position further upward and to the right than comparison examples 1 and 2 and comparison examples 5-10 in which B and Ti were not added. That is, although the hardness H₃ after hardening heat treatment is high, the impact ratio tends to be superior. This is conceivably because B increases the grain boundary strength, suppressing the precipitation of the coarse insoluble carbides, which triggers breakage. While B becomes the precipitation nucleus of insoluble carbides during softening heat treatment as described above and, as a result, conceivably suppresses the precipitation of coarse insoluble carbides which triggers breakage, the concentration of B is increased at the center part of an insoluble carbide 15, as shown in FIG. 8.

Further, compared to comparison example 11 as well in which B was added and Ti was not added, embodiments 1-10 comprise a high impact ratio with respect to the hardness H₃ after hardening heat treatment. In embodiments 1-10, Ti bonds with N before B, and thus the quantity of N that bonds with B conceivably decreases and the above described effect of B further increases.

Next, in terms of a wear mode, wear advances due to the occurrence and growth of a microcrack 21 from the front surface, and separation from the front surface. As shown in FIG. 9, the microcrack 21 preferentially propagates at the boundary between the insoluble carbide 22 and a matrix 23. That is, a concentration of stress conceivably occurs more readily at the boundary between the insoluble carbide 22 and the matrix 23, causing the microcrack 21 to more readily propagate, when the insoluble carbide 22 is of a larger size. Hence, as shown in FIG. 10, to summarize the relationship between the number of coarse insoluble carbides with a circle-equivalent diameter of 0.5 μm or greater and the wear ratio, the wear ratio decreases as this number decreases. That is, results reveal that decreasing the number of coarse insoluble carbides 22 makes it possible to improve wear resistance.

On the other hand, while the number of coarse insoluble carbides depends on the C content, a decrease in the C content causes a decrease in hardness as well as a decrease in wear resistance. As shown in FIG. 11, in a case where the hardness is 640 Hv or less after hardening heat treatment, the wear ratio rapidly increases, that is, the wear resistance greatly decreases. That is, there exists a lower limit of the C content required to provide favorable wear resistance as a steel belt element. On the other hand, in comparison example 1 and the like in which the C content is high, the cold punchability as a cold punching steel is unfavorable, and thus an upper limit of the C content also exists.

The range of the C content which provides both wear resistance as a steel belt element and cold punchability as a cold punching steel is 0.50-0.70% by mass, resulting in a hardness after hardening heat treatment of 640 Hv or greater and a number of coarse insoluble carbides with a circle-equivalent diameter of 0.5 μm or greater of 130 or less per 100 μm square, that is, approximately 1.3×10⁴ or less per 1 mm square.

Next, regarding the amount of B and coarse insoluble carbides, as shown in FIG. 12, after softening heat treatment, embodiment 10 in which B was added results in a greater number of insoluble carbides with a circle-equivalent diameter of 0.30 μm or less and a smaller number of even larger insoluble carbides, compared to comparison example 6 in which B was not added. That is, results reveal that, due to the amount of B, the insoluble carbides after softening heat treatment are refined.

Furthermore, as shown in FIG. 13, after hardening heat treatment, embodiment 10 in which B was added results in a greater number of insoluble carbides with a circle-equivalent diameter of 0.30 μm or less and a smaller number of even larger insoluble carbides, compared. to comparison example 6 in which B was not added. That is, after hardening heat treatment as well, the carbide refining effect resulting from the amount of B is effective.

As described above, without the amount of Mo or V and with a predetermined. component composition in which B and Ti were added as well as a predetermined heat treatment, insoluble carbides can be finely precipitated while increasing toughness, making it possible to obtain a steel for cold punching capable of providing a steel belt element superior in wear resistance as well.

Based on the above described embodiments and comparison examples, the component range of the steel for cold punching is defined using guidelines such as described below. First, C, Si, Mn, Cr, B, and Ti, which are the required additional elements, will be described.

C is the most important element for ensuring the wear resistance required as a steel belt element, as described above. When the amount of C is too small, the hardness after hardening heat treatment cannot be ensured, causing a decrease in wear resistance. On the other hand, when the amount of C is too large, coarse insoluble carbides remain after the hardening heat treatment, also causing a decrease in wear resistance. Further, when the amount of C is too large, carbides precipitate in film form at the grain boundary, decreasing grain boundary strength as well as toughness. Hence, as described above, C is within the range of 0.50-0.70% by mass.

Si is effective as a deoxidizing element of steel. When the amount of Si is too small, the steel cannot be sufficiently deoxidized. On the other hand, when the amount of Si is too large, the hardness after softening heat treatment increases, deteriorating cold punchability, which is required as a steel for cold punching. Hence, the Si content is within the range of 0.03-0.60% by mass.

Mn increases the hardenability of steel and is effective in ensuring the mechanical strength required as a steel belt element. When the amount of Mn is too small, hardenability cannot be ensured, resulting in a decrease in wear resistance, which is required as a steel belt element. On the other hand, when the amount of Mn is too large, cold punchability, which is required as a steel for cold punching, deteriorates. Hence, Mn is within the range of 0.50-1.00% by mass.

Cr, similar to Mn, increases the hardenability of steel and is effective in ensuring the mechanical strength required as a steel belt element. However, when the amount of Cr is too large, Cr readily dissolves in the iron carbide, causing the insoluble carbides to stabilize and the number of coarse insoluble carbides to increase. That is, wear resistance, which is required as a steel belt element, is decreased. Hence, Cr is within the range of 0.20-1.00% by mass.

B suppresses the gain boundary segregation of impurities, such as P, and increases grain boundary strength, and is thus effective in improving toughness, which is required WS a steel belt element. Further, as described above, since B is dispersed in the former cementite section in the pearlite phase, B becomes the precipitation nucleus of insoluble carbides precipitated during the softening heat treatment, causing the insoluble carbides to be finely dispersed and precipitated. With this arrangement, B has the effect of increasing wear resistance, which is required as a steel belt element. However, an increase in the amount of B increases costs. Hence, B is within the range of 0.0005-0.0050% by mass.

Ti preferentially bonds with N over B. becoming a Ti nitride, thereby suppressing formation of B nitrides and contributing to the improvements in grain boundary strength and wear resistance by B. When the amount of Ti is too small, formation of B nitrides cannot be sufficiently suppressed and thus improvements in the boundary strength and wear resistance by B are not achieved. On the other hand, an increase in the amount of Ti increases costs. Hence, Ti is within the range of 0.01-0.10% b mass.

Next, the optional additional elements will be described. For optional additional elements, the upper limit is defined within the range in which the characteristics as a steel belt element achieved by the above described required additional elements are not lost.

P decreases the strength of the grain boundary, but the decrease in this grain boundary strength is minor if the content is less than or equal to a certain value. Further, suppressing the amount can extend the refining process and cause an increase in cost as well. Hence, P is within the range of 0.025% or less by mass.

S bonds with Mn and generates an MnS inclusion, and thus, when included in excess, causes an increase in the inclusion amount which triggers a concentration of stress, leading to a decrease in fatigue strength, which is required as a steel belt element. However, if the S content is less than or equal to a certain value, the decrease in fatigue strength is extremely minor. Hence, S is within the range of 0.015% or less by mass.

Furthermore, the following describes Mo and V, which may be included as unavoidable impurities without being aggressively added.

Mo has the effect of suppressing the generation of film-like cementite on grain boundary, allowing expectation of further improvements in toughness when added. Further, Mo also has the effect of significantly increasing hardenability. However, the amount of Mo leads to a significant deterioration in punchability, which is required as a steel for cold punching, and an increase in costs. Further, according to the above described embodiments, Mo does not necessarily need to be added to achieve characteristics that are required as a steel belt element.

V forms fine V carbides in the steel and refines the grains, making it possible to improve toughness and wear resistance. However, the amount of V increases costs. Further, according to the above described embodiments, V does not necessarily need to be added to achieve characteristics that are required as a steel belt element.

Note that, since the Ac1 transformation temperature of the steel in the above described composition range is 714° C.-753° C., the maintained temperature of the softening heat treatment is preferably 700° C.-780° C.

While the above has described representative embodiments according to the present invention and modifications based thereon, the present invention is not necessarily limited thereto. Those skilled in the art may find various alternative embodiments and adaptations without deviating from the scope of the appended claims.

DESCRIPTION OF THE REFERENCE NUMERALS

-   10 wear tester -   11 ring -   13 wear test piece -   21 microcrack -   22 insoluble carbide -   23 matrix 

1. A steel for cold punching comprising a steel of a component composition that satisfies 10.8 [C]+5.6 [Si]+2.7 [Mn]+0.3 [Cr]+7.8 [Mo]+1.4 [V]≦13, given [M] as the percent by mass of the chemical element M, further comprising: a component composition of C within the range of 0.50 to 0.70%, Si within the range of 0.03 to 0.60%,. Mn within the range of 0.50 to 1.00%, Cr within the range of 0.20 to 1.00%. Ti within the range of 0.01 to 0.10%, and B within the range of 0.0005 to 0.0050% by mass as required additional elements, P within the range of 0.025% or less and S within the range of 0.015% or less by mass as optional additional elements, and the remainder as Fe and unavoidable impurities, wherein: a hardness of 88 HRB or less is provided in a structure achieved by heating and maintaining the steel in an austenite single-phase temperature range and subsequently performing cooling at a predetermined rate to disperse fine carbides in a mainly ferrite+pearlite mixed structure.
 2. The steel for cold punching according to claim 1 wherein the steel is characterized by the suppression of coarse carbides with a circle-equivalent diameter of 0.5 μm or greater to 1.2×10⁵ carbides or less per 1 mm square in a sectional structure.
 3. An element for steel belts wherein the element is characterized by the provision of a quenching and tempering heat treatment after the steel for cold punching according to claim 1 is cold punched into a predetermined shape, thereby providing a hardness of 640 Hv or greater.
 4. The element for steel belts according to claim 3 wherein the steel element is characterized by the suppression of coarse carbides with a circle-equivalent diameter of 0.5 μm or greater to 1.3×10⁴ carbides or less per 1 mm square in a sectional structure.
 5. An element for steel belts wherein the element is characterized by the provision of a quenching and tempering heat treatment after the steel for cold punching according to claim 2 is cold punched into a predetermined shape, thereby providing a hardness of 640 Hv or greater. 